1. Field of the Invention
This invention relates to data compression, and more particularly to data compression using adaptive bit allocation and hybrid lossless entropy encoding.
2. Description of Related Art
A number of methods exist for compressing data. These methods are characterized as either "lossy" or "lossless". Lossy compression can achieve substantial compression ratios by, in effect, omitting redundant or unimportant information while still maintaining a reasonable semblance to the original data. Since data is lost, lossy compression is normally used only for data, such as sound or graphics (still or motion), that can be perceived reasonably accurately despite the loss of information. Lossless compression generally results in lower compression ratios but can reconstitute the original data in its entirety. Thus, lossless compression is generally used for machine code, text, and numerical data that require perfect precision.
A number of different methods exist for performing lossy compression, including the well-known JPEG standard for graphics and MPEG standard for video. However, no one particular compression algorithm has been shown to be optimal for all data content. Matching an algorithm to particular data (e.g., graphics images) can achieve higher compression ratios. In some contexts, such as transmission of data over a constrained bandwidth (e.g., typical modems, LANs, or WANs), an improvement in compression ratio by even a few percent can achieve substantial long-term savings.
Accordingly, a need exists for improved data compression algorithms, particularly for lossy compression of graphics images. The present invention provides such an improved lossy compression algorithm that is particularly efficient for compressing graphics images.